ANS-C01 Network Security, Compliance and Governance Practice Question
A company has deployed a multi-tier application in a VPC with public and private subnets. The web tier runs on EC2 instances in public subnets, and the application tier runs on EC2 instances in private subnets. The application tier must only accept traffic from the web tier security group. The security group for the application tier has an inbound rule allowing HTTP traffic from the web tier security group. However, the application team reports that the web tier instances cannot connect to the application tier instances. The network administrator has verified that the web tier instances can resolve the private DNS names of the application tier instances, and the route tables are correctly configured. What is the MOST likely cause of the connectivity issue?
⚠ Common exam trap
AWS often tests the misconception that security groups are stateless like network ACLs, leading candidates to incorrectly assume that outbound rules on the destination security group are needed for return traffic, when in fact the issue is the outbound rules on the source security group.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The web tier security group's outbound rules do not allow traffic to the application tier security group.
Security groups are stateful, meaning that if you allow inbound traffic, the return traffic is automatically allowed regardless of outbound rules. However, the outbound rules of the initiating security group (the web tier) must permit the traffic to reach the destination. Since the web tier instances initiate the connection to the application tier, the web tier security group's outbound rules must allow HTTP traffic to the application tier security group. If those outbound rules are missing or too restrictive, the connection will fail even though the application tier's inbound rule is correct.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The application tier instances do not have a route to the web tier instances.
Why it's wrong here
Route tables are correctly configured per the scenario, and DNS resolution works.
- ✗
The application tier security group's inbound rule is stateful, but the outbound rule is not configured to allow return traffic.
Why it's wrong here
Security groups are stateful; outbound return traffic is automatically allowed.
- ✓
The web tier security group's outbound rules do not allow traffic to the application tier security group.
Why this is correct
If the web tier security group does not have an outbound rule allowing traffic to the application tier, connections will fail.
- ✗
The network ACL for the private subnet is blocking inbound traffic from the public subnet.
Why it's wrong here
NACLs are stateless; if they are not configured correctly, they could block traffic, but the question does not indicate NACL misconfiguration.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.